The immune system is a closed network of interacting elements and the expression of immunocompetence involves cellular interactions among complementary (idiotypic and anti-idiotypic) cells. In this application I discuss why regulatory cellular interactions between idiotypic and anti-idiotypic cells can best be studied in a well-defined antigenic system. Phosphorylcholine antigens induce both in vivo and in vitro idiotypically homogeneous humoral responses. I propose a transplantation model to study idiotype-specific interactions and their role in the establishment and maintenance of idiotypic clonal dominance.